It is well known that bearing plates facilitate an increase of the number of adjusting angular positions of the back portion of a seat with respect to the seating portion of the seat, particularly when the fixed toothing of the movable ring and the toothing of the bearing plates are both very small.
However the known articulartion members have various drawbacks, and particularly rather important plays because of the manufacture of the parts, which results in an angular displacement of the back portion which is bad for the comfort of the seat user.
These known articulation members which position the back portion with respect to the seating portion are based on a positioning of the bearing plates in such a manner that teeth which are made on the toothed ring and on the toothed bearing plates are placed opposite the one from the other with a shift of an angle equal to that of a tooth. It is therefore impossible or at least not very possible, by using these articulation members to compensate the plays and, moreover, the minimum angle between two positions of the back portion is equal to the angular pitch of one tooth.